1. Field of the Invention
The present invention relates to a current driver circuit for driving a current-driven element such as an organic EL (electroluminescent) element, and to an image display device that both incorporates this type of current driver circuit and uses a current-driven element as a luminous element.
2. Description of the Related Art
In recent years, devices using current-driven luminous elements such as organic EL elements have been receiving increasing attention for use as image display devices used in portable telephones or the output devices of computers. Organic EL elements are also called xe2x80x9corganic light-emitting diodesxe2x80x9d and have the advantage of allowing drive by direct current (dc). When organic EL elements are used in a display device, organic EL elements for each picture element (pixel) are typically arranged in matrix form on a substrate to constitute a display panel. As the construction of a display device, an active matrix arrangement is under investigation in which TFTs (thin-film transistors) are formed on this substrate and the organic EL elements of respective picture elements are driven by way of the TFTS.
Since an organic EL element is a current-driven element, however, driving an organic EL element by a TFT precludes the use of a circuit configuration that is the same as an active matrix liquid crystal display device that uses liquid crystal cells, which are voltage-driven elements. Conventionally, active matrix drive circuits have been proposed in which organic EL elements and TFTs, which are MOS (metal-oxide semiconductor) transistors, are connected in a series and inserted between a power supply line and a ground line so as to allow the application of a control voltage to the gates of the TFTS, and further, in which holding capacitors that retain this control voltage are connected to the gates of the TFTs with switching elements provided between the TFTs and signal lines for applying the control voltage to respective picture elements. In such a circuit, the control voltage is outputted in a time-division manner to each picture element on the signal lines, and each switching element is controlled to enter a conductive state (ON state) only at the timings at which the control voltage is outputted to the corresponding picture elements. Thus, when a switching element enters the conductive state, the control voltage at that time is applied to the gates of the TFTS, whereby a current that accords with the control voltage flows through the organic EL element and the holding capacitor is charged by this control voltage. If the switching element transits to the cut-off state (OFF state) in this state, the already applied control voltage continues to be applied to the gates of the TFTs under the effect of the holding capacitor, and a current that accords with this control voltage therefore continues to flow to the organic EL element. This type of the circuit is disclosed in, for example, W099/65011.
In this circuit of the prior art, however, the occurrence of variations in the characteristics of the TFT brings about variations in the current that flows to the organic EL element of each picture element despite the application of the same control voltage, and these variations therefore prevent the realization of a suitable display, particularly when performing a gray-scale display. In addition, the occurrence of voltage drops on the fine signal lines also results in variations in the current that flows to organic EL elements.
In the interest of solving the above-described problems when constituting an active matrix display device, the assignee of this invention has previously proposed in Japanese Patent Laid-open Application No. 11-282419 (JP, 11282419, A), which corresponds to U.S. Pat. No. 6,091,203 of Kawashima et al., a current driver circuit that is directed toward driving current-driven active elements such as the organic EL elements that constitute the picture elements of this type of display device. FIG. 1 is a circuit diagram showing the basic circuit configuration of the current driver circuit proposed in JP, 11282419, A. This figure shows the circuit of one picture element.
The circuit shown in FIG. 1 is arranged such that signal current on signal line 53 is converted, by means of a current mirror circuit composed of n-channel transistors 56 and 58, to a driving current that flows to organic EL element 61, and such that organic EL element 61 is driven at a constant current by the driving current that accords with the signal current. Power-supply line 51 and ground line 52 are provided, the power supply voltage being positive, the anode of organic EL element 61, which is provided as the load of transistor 58, is connected to power-supply line 51, and the cathode of organic EL element 61 is connected to the drain of transistor 58. The sources of transistors 56 and 58 are each connected to ground line 52. The gate and drain of transistor 56 are connected to each other and further connected to the gate of transistor 58 by way of switch element 62. Holding capacitance 60 is provided between the gate of transistor 58 and ground line 52. The drain of transistor 56 is connected to signal line 53 by way of switch element 63. Switch elements 62 and 63 are constituted by, for example, MOS switches, and the control terminals of each are connected to selection line 54. If MOS transistors are used for switch elements 62 and 63, the control terminals are the gate terminals the MOS transistors.
When selection line 54 become active and switch elements 62 and 63 become conductive, the signal current supplied from signal line 53 flows to transistor 56 that is diode-connected by way of switch element 63, and holding capacitor 60 is charged until the voltage across both ends of holding capacitor 60 reaches the gate-to-source voltage of transistor 56. Since transistors 56 and 58 constitute a current mirror circuit, a current that has the same magnitude as the signal current from signal line 53 flows to transistor 58 if the channel length and channel width of transistors 56 and 58 are the same, and this current flows to organic EL element 61, which is the load of transistor 58.
When selection line 54 becomes inactive and switch elements 62 and 63 enter the cutoff state, the signal current is not supplied from signal line 53 because switch element 63 is in the cutoff state, but the voltage level in holding capacitor 60 that is connected to the gate of transistor 58 remains at the same value as when switch elements 62 and 63 were in the conductive state because switch element 62 is in the cutoff state, and transistor 58 therefore continues to direct to organic EL element 61 a current of the same value as when switch elements 62 and 63 were conductive.
In this circuit, causing a signal current to flow instead of applying a control voltage to the signal line can curtail the effect of voltage drops in the signal line, and using a current mirror circuit allows a driving current to be obtained that accords with the signal current and that is unaffected by differences in transistor characteristics between the picture elements.
Nevertheless, in contrast with transistors formed on a single-crystal silicon semiconductor substrate, when the transistors that make up the above-described current driver circuit are constituted by amorphous silicon TFTS (thin-film transistors) or polycrystalline silicon TFTS, variations in threshold voltage Vth on the order of several tens of millivolts may occur even when these TFTS are arranged contiguous to each other. Thus, despite the contiguous arrangement of transistors 56 and 58 that make up the current mirror circuit in the circuit shown in FIG. 1, variations in threshold are difficult to suppress and matching of the two transistors 56 and 58 is therefore difficult to achieve. In addition to variations in threshold value, variations in the carrier mobility or gate oxide film thickness of the transistor may also prevent matching of the transistors that make up a current mirror circuit. Variations in the threshold value, carrier mobility, and gate oxide film thickness prevent matching between transistors and result in large variations in the input/output characteristic of the current mirror circuit.
The circuit shown in FIG. 1 is of a configuration for transferring the signal current that is supplied from signal line 53 to organic EL element 61, which is the load, by way of a current mirror circuit made up by transistors 56 and 58, but failure to achieve matching of the gate-to source voltages of transistors 56 and 58 as described in the foregoing explanation prevents accurate transfer of the signal current from signal line 53 to organic EL element 61. FIG. 2 shows the input/output transfer characteristic of the current mirror circuit when the threshold values Vth of the two transistors 56 and 58 that constitute the current mirror circuit each vary by 50 mV. The channel length and channel width of each of transistors 56 and 58 is 4 xcexcm. The inclined line shown in the center of the graph represents the transfer characteristic when no variation occurs in the threshold value, and the lines on either side of this line represent the transfer characteristics when variations occur in the threshold value. As shown in FIG. 2, when threshold value Vth varies by approximately xc2x150 mV, the output current, i.e., the current that flows to the organic EL element, varies by approximately xc2x113%.
Thus, in the current driver circuit shown in FIG. 1 as well, when TFTs are used to constitute a circuit that is applied in an organic EL picture display device, there remain various problems to be solved, such as the occurrence of gray-scale error between picture elements which results in a decrease in picture quality in the display panel, and further, a drop in production yield and the consequent increase in cost.
It is an object of the present invention to provide a current driver circuit that is suitable for, for example, an organic EL image display device and that mitigates the influence of variations between the transistors that make up a current mirror circuit while using the current mirror circuit.
It is another object of the present invention to provide an image display device having this type of current driver circuit.
The present invention relates to a current driver circuit that uses a current mirror circuit as described in the foregoing explanation. Although current mirror circuits exist in various forms, a basic configuration is provided with: a first transistor for generating a gate potential that accords with the drain current and a second transistor having its drain connected to a current-driven element and that is configured such that a potential that accords with the gate potential of the first transistor is applied to the gate of the second transistor. By means of this basic configuration, when a signal current is caused to flow to the first transistor, the second transistor drives the current-driven element by means of a drain current that accords with the signal current. In the present invention, such a current mirror circuit is provided with: a third transistor that has its gate connected to the gate of the first transistor, that is connected in a series to the source of the first transistor, and that operates in a non-saturation region (linear region); and a fourth transistor that has its gate connected to the gate of the second transistor, that is connected in a series to the source of the second transistor, and that operates in a non-saturation region. The provision of this third and fourth transistor mitigates the influence of variations between the transistors that make up the current mirror circuit. In this case, the third and fourth transistors essentially function as resistance.
The method of arranging the third and fourth transistors in the present invention is open to various modifications according to differences in the form and configuration of the current mirror circuit, and actual examples of these arrangements will be clarified by embodiments of the present invention that are described hereinbelow.
Essentially, the object of the present invention is realized by a current driver circuit that includes: a current mirror circuit that includes at least a first transistor and a second transistor, the first transistor generating a gate potential that accords with the drain current, and a second transistor having its drain connected to a current-driven element wherein the application of a potential that accords with the gate potential of the first transistor to the gate of the second transistor causes the second transistor to drive the element at a current that corresponds to the drain current of the first transistor; a holding capacitor for holding the gate potential of the second transistor; a first switch element for connecting the drain of the first transistor to a signal line that provides a signal current in accordance with a received control signal; a second switch element that enters either a conductive or cutoff state in accordance with a received control signal and that causes the current mirror circuit to operate when in the conductive state and both prevents operation of the current mirror circuit and cuts off the charge/discharge path from the holding capacitor when in the cutoff state; a third transistor that is inserted between the source of the first transistor and a line that supplies the source currents of the first and second transistors, and that operates within a non-saturation region; a fourth transistor that is inserted between the source of the second transistor and the line that supplies the source currents of the first and second transistors, and that operates in a non-saturation region.
In the present invention, connecting transistors, which operate in a non-saturation region (linear region) and that essentially function as resistance, to the transistors that constitute the current mirror circuit enables suppression of variations between the input and output currents of the current mirror circuit and allows a current driver circuit to be obtained that can drive an element accurately based on a signal current. Application of the present invention therefore allows an improvement in the picture quality of a display image in, for example, an organic EL display device.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, which illustrate examples of the present invention.